Thursday, September 26, 2013

The process of multiplexing is carried out at the
transport layer. Several conversations are multiplexed in to one connection or
physical links or virtual circuit. For example, suppose the host has only one
network address available for use. Then it has to be used by all the transport
connections originating at that host. For multiplexing the following two main
strategies are followed:

ØUpward
multiplexing and

ØDownward
multiplexing

Upward Multiplexing

- In upward multiplexing, the different transport
connections are multiplexed in to one network connection.

- These transport
connections are grouped by the transport layer as per their destinations.

- It
then maps the groups with the minimum number of network connections possible.

- The upward multiplexing is quite useful where the network connections come very
expensive.

Downward Multiplexing

- It is only
used when the connections with high bandwidth are required.

- In case of the
downward multiplexing, the multiple network connections are opened by the
transport layer and the traffic is distributed among them.

- But for using
downward multiplexing, it is necessary that this capacity must be handled well
by the subnet’s data links.

Another Technique

- In either of the cases it is not guaranteed that
the segments will be delivered in order.

- Therefore, another technique is adopted.

- The segments are numbered sequentially.

- Each octet is numbered by the TCP
sequentially.

- Segments are then numbered based up on the number of the first
octet present in that segment.

- The segments might get damaged in the transition
or some may even fail to arrive at the destination.

- This failure is not
acknowledged by the transmitter.

- However, the successful receipt of the segment
is does acknowledged by the receiver.

- Sometimes, the cumulative acknowledgements
might be used.

- If the ACK triggers a time out interrupt, the re-transmission
of the segment is done.

- Also the re-transmission is done when an ACK is lost.

- The receiver must have the ability to recognize the duplicate ACKs.

- If such
thing occurs, the receiver assumes by itself the ACK might have been lost.

- This
happens when the ACK duplicate is received before the connection is closed.

- If
the duplicate is received after the closure of the connection, the situation is
dealt differently.

- In this case, the sender and receiver are allowed to know
about each other’s existence.

- They negotiate about the parameters and the
transport entity resources are allocated based up on some mutual agreement.

The
connection release is of two types:

ØAsymmetric
release:

This is the one used in the telephone systems. However it does not
works well for the network that use packet switching.

ØSymmetric
release:

- This is certainly better than the previous one.

- Here, all the
directions are released independently with respect to each other.

- The host
continues receiving data after the disconnection TPDU has been sent.

- But the
symmetric release has another problem which is related with indirection levels
and fake messages.

- There are no proper solutions for this problem in case of
the unreliable communication media.

- Note that this has nothing to do with the
protocol.

- Putting a reliable protocol over an unreliable medium can actually
guarantee the delivery of the message.

- Another thing to be noted is that it the
time limit within which the message will be delivered cannot be guaranteed by
any protocol.

- Error conditions might prolong the delivery period.

- Restarting
the connections can lead to the loss of all the state info and the connection
might remain as half-open.

- Since no protocol has been designed to deal with
this problem therefore one has to go forward with the risks associated with
releasing the connections.